Natural succession of grassland on the Loess Plateau of China affects multifractal characteristics of soil particle-size distribution and soil nutrients

Knowledge of the changes of soil properties and soil particle-size distribution (PSD) is important for quantifying soil quality and dynamics during natural succession. We applied multifractal theory to characterize the PSDs in five soil layers of nine croplands abandoned for less than 1 year, 2, 5, 7, 10, 15, 20, 30 and 35 years and evaluated the changes of soil organic carbon (SOC), total nitrogen (total N), total phosphorus (total P), soil texture and multifractal parameters [capacity dimension (D ₀), entropy dimension (D ₁), correlation dimension (D ₂) and Hölder exponent of order zero (α ₀)] during natural succession on the Loess Plateau of China. Rényi spectra (D _q ) and singularity spectra f(α) characterized the PSDs well and sensitively reflected the changes of heterogeneity of the surface soil (0–20 cm) during natural succession. SOC, total N and clay contents and the multifractal parameters increased significantly by the late stage of succession, mostly in the topsoil (0–10 cm). Natural succession thus effectively improved the soil physicochemical properties on the Loess Plateau of China, even though it was time-consuming. SOC and total N contents decreased with depth throughout the natural succession, but the multifractal parameters were higher in the topsoil only in the late stage. D ₁ and D ₂ were strongly and positively correlated with SOC and total N contents in the surface layers and with fine particles in all layers, suggesting that D ₁ and D ₂ may be sensitive and practical indices for quantifying changes in soil properties and erosion.     

Non-native liana, <Emphasis Type="Italic">Euonymus fortunei</Emphasis>, associated with increased soil nutrients,unique bacterial communities,and faster decomposition rate

Invasive plants have wide-ranging impacts on native systems including reducing native plant richness and altering soil chemistry, microbes, and nutrient cycling. Increasingly, these effects are found to linger long after removal of the invader. We examined how soil chemistry, bacterial communities, and litter decomposition varied with cover of Euonymus fortunei, an invasive evergreen liana, in two central Kentucky deciduous forests. In one forest, E. fortunei invaded in the late 1990s but invasion remained patchy and we paired invaded and uninvaded plots to examine the associations between E. fortunei cover and our response variables. In the second forest, E. fortunei had completely invaded the forest by 2005; areas where it had been selectively removed by 2010 were paired with an adjacent invaded plot. Where E. fortunei had patchily invaded, E. fortunei patches had up to 3.5× nitrogen, 2.7× carbon, and 1.9× more labile glomalin in soils than uninvaded plots, whereas there were no differences in soil characteristics between invaded and removal plots. In the patchily invaded forest, bacterial community composition varied among invaded and non-invaded plots, whereas bacterial communities did not vary among invaded and removal plots. Finally, E. fortunei leaf litter decomposed faster (k = 4.91 year^?1) than the native liana (k = 3.77 year^?1), Vitis vulpina; decomposition of both E. fortunei and V. vulpina was faster in invaded (k = 7.10 year^?1) than removal plots (k = 4.77 year^?1). Our findings suggest that E. fortunei invasion increases the rate of leaf litter decomposition via high-quality litter, alters the decomposition environment, and shifts in the soil biotic communities associated with a dense mat of wintercreeper. Land managers with limited resources should target the densest mats for the greatest restoration potential and remove wintercreeper patches before they establish dense mats.     

Factors controlling decomposition rates of fine root litter in temperate forests and grasslands

Background and aims

Fine root decomposition contributes significantly to element cycling in terrestrial ecosystems. However, studies on root decomposition rates and on the factors that potentially influence them are fewer than those on leaf litter decomposition. To study the effects of region and land use intensity on fine root decomposition, we established a large scale study in three German regions with different climate regimes and soil properties. Methods In 150 forest and 150 grassland sites we deployed litterbags (100 μm mesh size) with standardized litter consisting of fine roots from European beech in forests and from a lowland mesophilous hay meadow in grasslands. In the central study region, we compared decomposition rates of this standardized litter with root litter collected on-site to separate the effect of litter quality from environmental factors.

Results

Standardized herbaceous roots in grassland soils decomposed on average significantly faster (24?±?6 % mass loss after 12 months, mean ± SD) than beech roots in forest soils (12?±?4 %; p?Conclusions Grasslands, which have higher fine root biomass and root turnover compared to forests, also have higher rates of root decomposition. Our results further show that at the regional scale fine root decomposition is influenced by environmental variables such as soil moisture, soil temperature and soil nutrient content. Additional variation is explained by root litter quality.     

Effects of litter types, microsite and root diameters on litter decomposition in Pinus sylvestris plantations of northern China

Background and aims

Litter decomposition is a major process in the carbon (C) flow and nutrient cycling of terrestrial ecosystems, but the effects of litter type, microsite, and root diameter on decomposition are poorly understood.

Methods

Litterbags were used to examine the decomposition rate of leaf litter and roots at three soil depths (5, 10 and 20 cm) over a 470-day period in Pinus sylvestris plantations in northern China.

Results

Leaves and the finest roots decomposed more quickly at 5 cm depth and coarser roots (>1-mm) decomposed more quickly at 10 and 20 cm depth. Roots generally decomposed more quickly than leaf litter, except at 5 cm deep; leaves decomposed more quickly than the coarsest roots (>5-mm). Root decomposition was strongly influenced by root diameter. Leaves experienced net nitrogen (N) immobilization and coarse roots (>2-mm) experienced more N release than fine roots. Significant heterogeneity was seen in N release for fine-roots (<2-mm) with N immobilization occurring in smaller (0.5–2-mm) roots and N release in the finest roots (<0.5-mm).

Conclusions

Soil depth of litter placement significantly influenced the relative contribution of the decomposition of leaves and roots of different diameters to carbon and nutrient cycling.     

Impact of <Emphasis Type="Italic">Heterobasidion</Emphasis> root-rot on fine root morphology and associated fungi in <Emphasis Type="Italic">Picea abies</Emphasis> stands on peat soils

We examined differences in fine root morphology, mycorrhizal colonisation and root-inhabiting fungal communities between Picea abies individuals infected by Heterobasidion root-rot compared with healthy individuals in four stands on peat soils in Latvia. We hypothesised that decreased tree vitality and alteration in supply of photosynthates belowground due to root-rot infection might lead to changes in fungal communities of tree roots. Plots were established in places where trees were infected and in places where they were healthy. Within each stand, five replicate soil cores with roots were taken to 20 cm depth in each root-rot infected and uninfected plot. Root morphological parameters, mycorrhizal colonisation and associated fungal communities, and soil chemical properties were analysed. In three stands root morphological parameters and in all stands root mycorrhizal colonisation were similar between root-rot infected and uninfected plots. In one stand, there were significant differences in root morphological parameters between root-rot infected versus uninfected plots, but these were likely due to significant differences in soil chemical properties between the plots. Sequencing of the internal transcribed spacer of fungal nuclear rDNA from ectomycorrhizal (ECM) root morphotypes of P. abies revealed the presence of 42 fungal species, among which ECM basidiomycetes Tylospora asterophora (24.6 % of fine roots examined), Amphinema byssoides (14.5 %) and Russula sapinea (9.7 %) were most common. Within each stand, the richness of fungal species and the composition of fungal communities in root-rot infected versus uninfected plots were similar. In conclusion, Heterobasidion root-rot had little or no effect on fine root morphology, mycorrhizal colonisation and composition of fungal communities in fine roots of P. abies growing on peat soils.     

Species and Genotype Effects of Bioenergy Crops on Root Production,Carbon and Nitrogen in Temperate Agricultural Soil

Bioenergy crops have a secondary benefit if they increase soil organic C (SOC) stocks through capture and allocation below-ground. The effects of four genotypes of short-rotation coppice willow (Salix spp., ‘Terra Nova’ and ‘Tora’) and Miscanthus (M.?×?giganteus (‘Giganteus’) and M. sinensis (‘Sinensis’)) on roots, SOC and total nitrogen (TN) were quantified to test whether below-ground biomass controls SOC and TN dynamics. Soil cores were collected under (‘plant’) and between plants (‘gap’) in a field experiment on a temperate agricultural silty clay loam after 4 and 6 years’ management. Root density was greater under Miscanthus for plant (up to 15.5 kg m^?3) compared with gap (up to 2.7 kg m^?3), whereas willow had lower densities (up to 3.7 kg m^?3). Over 2 years, SOC increased below 0.2 m depth from 7.1 to 8.5 kg m^?3 and was greatest under Sinensis at 0–0.1 m depth (24.8 kg m^?3). Miscanthus-derived SOC, based on stable isotope analysis, was greater under plant (11.6 kg m^?3) than gap (3.1 kg m^?3) for Sinensis. Estimated SOC stock change rates over the 2-year period to 1-m depth were 6.4 for Terra Nova, 7.4 for Tora, 3.1 for Giganteus and 8.8 Mg ha^?1 year^?1 for Sinensis. Rates of change of TN were much less. That SOC matched root mass down the profile, particularly under Miscanthus, indicated that perennial root systems are an important contributor. Willow and Miscanthus offer both biomass production and C sequestration when planted in arable soil.     

Tree species mixture inhibits soil organic carbon mineralization accompanied by decreased r-selected bacteria

Background and aims

Fine-root functioning is a major driver of plant growth and strongly influences the global carbon cycle. While fine-root over-yielding has been shown in the upper soil layers of mixed-species forests relative to monospecific stands, the consequences of tree diversity on fine-root growth in very deep soil layers is still unknown. Our study aimed to assess the consequences of mixing Acacia mangium and Eucalyptus grandis trees on soil exploration by roots down to the water table at 17 m depth in a tropical planted forest.

Method

Fine roots (diameter < 2 mm) were sampled in a randomized block design with three treatments: monospecific stands of Acacia mangium (100A), Eucalyptus grandis (100E), and mixed stands with 50% of each species (50A50E). Root ingrowth bags were installed at 4 depths (from 0.1 m to 6 m) in the three treatments within three different blocks, to study the fine-root production over 2 periods of 3 months.

Results

Down to 17 m depth, total fine-root biomass was 1127 g m^?2 in 50A50E, 780 g m^?2 in 100A and 714 g m^?2 in 100E. Specific root length and specific root area were 110–150% higher in 50A50E than in 100A for Acacia mangium trees and 34% higher in 50A50E than in 100E for Eucalyptus grandis trees. Ingrowth bags showed that the capacity of fine roots to explore soil patches did not decrease down to a depth of 6 m for the two species.

Conclusions

Belowground interactions between Acacia mangium and Eucalyptus grandis trees greatly increased the exploration of very deep soil layers by fine roots, which is likely to enhance the uptake of soil resources. Mixing tree species might therefore increase the resilience of tropical planted forests through a better exploration of deep soils.

     

Further evidence for slow decomposition of very fine roots using two methods: litterbags and intact cores

Background and aims

Root decomposition studies have rarely considered the heterogeneity within a fine-root system. Here, we investigated fine root (< 0.5 and 0.5–2 mm in diameter) decomposition and accompanying nutrient dynamics of two temperate tree species—Betula costata Trautv and Pinus koraiensis Sieb. et Zucc.

Methods

Both litterbag and intact-core techniques were used to examine decomposition dynamic and nutrient release of the two size class roots over a 498-day period. Moreover, we examined differences between the two approaches.

Results

The very fine roots (< 0.5 mm) with an initially lower C:N ratio, decomposed more slowly than 0.5–2 mm roots of both tree species. The differences in mass loss between size classes were smaller when using the intact-core technique compared with litterbag technique. In contrast to root biomass loss, net N release was much higher in the fine roots (< 0.5 mm). All fine roots initially released N (0–75 days), but immobilized N to varying extent in the following days (75–498 days) during decomposition.

Conclusions

Our results suggest that the slow decomposition rate of very fine roots (< 0.5 mm) may be determined by their high concentration of acid-unhydrolyzable structural components. Additionally, the heterogeneity within a bulk fine-root system could lead to differences in their contribution to soil in terms of carbon and nitrogen dynamics.     

Reduced Winter Snowpack and Greater Soil Frost Reduce Live Root Biomass and Stimulate Radial Growth and Stem Respiration of Red Maple (<Emphasis Type="Italic">Acer rubrum</Emphasis>) Trees in a Mixed-Hardwood Forest

Northeastern U.S. forests are currently net carbon (C) sinks, but rates of C loss from these ecosystems may be altered by the projected reduction in snowpack and increased soil freezing over the next century. Soil freezing damages fine roots, which may reduce radial tree growth and stem respiration. We conducted a snow removal experiment at Harvard Forest, MA to quantify effects of a reduced winter snowpack and increased soil freezing on root biomass, stem radial growth and respiration in a mixed-hardwood forest. The proportion of live fine root biomass during spring (late-April) declined with increasing soil frost severity (P = 0.05). Basal area increment index was positively correlated with soil frost severity for Acer rubrum, but not Quercus rubra. Rates of stem respiration in the growing season correlated positively with soil frost duration in the previous winter, (\( R^{2}_{{{\text{LMM}}({\text{m}})}} \) = 0.15 and 0.24 for Q. rubra and A. rubrum, respectively). Losses of C from stem respiration were comparable to or greater than C storage from radial growth of Q. rubra and A. rubrum, respectively. Overall, our findings suggest that in mixed-hardwood forests (1) soil freezing has adverse effects on spring live root biomass, but at least in the short-term could stimulate aboveground processes such as stem respiration and radial growth for A. rubrum more than Q. rubra, (2) stem respiration is an important ecosystem C flux and (3) the increasing abundance of A. rubrum relative to Q. rubra may have important implications for C storage in tree stem biomass.     

Competition between native Antarctic vascular plants and invasive <Emphasis Type="Italic">Poa annua</Emphasis> changes with temperature and soil nitrogen availability

Over the last decades human have introduced non-native organisms to Antarctica, including the grass species Poa annua. This non-native grass under constant growth temperatures has been shown negatively affect the growth of the only two native Antarctic vascular plants, Deschampsia antarctica and Colobanthus quitensis, under constant growth temperatures. However, whether there are changes in the interaction between these species under warmer conditions is an important question. In cold ecosystems, soil nutrient status directly affects plant responses to increases in temperature and Antarctic soils are highly variable in nutrient supply. Thus, in this study we experimentally assessed the interaction between the non-native Poa with the two native Antarctic vascular plant species at two different temperatures and levels of nutrient availability. Individual mats of the study species were collected in King George Island, and then transported to Concepcion where we conducted competition experiments. In the first experiment we used soil similar to that of Antarctica and plants in competition were grown at two temperatures: 5°/2° and 11°/5 °C (day/night temperature). In a second experiment plants were grown in these two temperature regimes, but we varied nitrogen (N) availability by irrigating plants with Hoagland solutions that contained 8000 or 300 µM of N. Overall, Poa exerted a competitive effect on Deschampsia but only at the higher temperature and higher N availability. At 5°/11 °C the competitive response of Deschampsia to Poa was of similar magnitude to the competitive effect of P. Deschampsia, and the competitive effect was greater with at low N. The competitive effect of Poa was similar to the competitive response of Colobanthus to Poa at both temperatures and N levels. Thus, at low temperatures and N soil content the native Antarctic species might withstand Poa invasion, but this might change with climate warming.     

Deep rooting and global change facilitate spread of invasive grass

Abiotic global change factors, such as rising atmospheric CO₂, and biotic factors, such as exotic plant invasion, interact to alter the function of terrestrial ecosystems. An invasive lineage of the common reed, Phragmites australis, was introduced to North America over a century ago, but the belowground mechanisms underlying Phragmites invasion and persistence in natural systems remain poorly studied. For instance, Phragmites has a nitrogen (N) demand higher than native plant communities in many of the ecosystems it invades, but the source of the additional N is not clear. We exposed introduced Phragmites and native plant assemblages, containing Spartina patens and Schoenoplectus americanus, to factorial treatments of CO₂ (ambient or +300 ppm), N (0 or 25 g m^?2 year^?1), and hydroperiod (4 levels), and focused our analysis on changes in root productivity as a function of depth and evaluated the effects of introduced Phragmites on soil organic matter mineralization. We report that non-native invasive Phragmites exhibited a deeper rooting profile than native marsh species under all experimental treatments, and also enhanced soil organic matter decomposition. Moreover, exposure to elevated atmospheric CO₂ induced a sharp increase in deep root production in the invasive plant. We propose that niche separation accomplished through deeper rooting profiles circumvents nutrient competition where native species have relatively shallow root depth distributions; deep roots provide access to nutrient-rich porewater; and deep roots further increase nutrient availability by enhancing soil organic matter decomposition. We expect that rising CO₂ will magnify these effects in deep-rooting invasive plants that compete using a tree-like strategy against native herbaceous plants, promoting establishment and invasion through niche separation.     

Fine root production in three zones of secondary mangrove forest in eastern Thailand

Key message

High root productions, especially in the fine roots, estimated by ingrowth cores were confirmed in mangrove forests. The zonal variation in root production was caused by inundation regime and soil temperature.

Abstract

Mangrove forests have high net primary productivity (NPP), and it is well known that these trees allocate high amounts of biomass to their root systems. In particular, fine root production (FRP) comprises a large component of the NPP. However, information on root production remains scarce. We studied FRP in three zones (Avicennia, Rhizophora, and Xylocarpus) of a mangrove forest in eastern Thailand using ingrowth cores (0–30 cm of soil depth). The root biomass and necromass were periodically harvested from the cores and weighed during the one-year study. The FRP was determined by summation of the fine root biomass (FRB) and root necromass. The results showed that the FRB clearly increased in the wet and cool dry seasons. Magnitude of FRB in the Rhizophora and Xylocarpus zones was 1171.07 and 764.23 g/m²/30 cm, respectively. The lowest FRB (292.74 g/m²/30 cm) was recorded in the Avicennia zone locating on the river edge where there is a greater frequency of inundation than the other zones. Root necromass was high in the Rhizophora and Xylocarpus zones, and accumulated noticeably when soil temperatures rapidly declined during the middle of the wet season to cool dry season. However, root necromass in the Avicennia zone varied within a small range. We attributed the small accumulation of root necromass in the Avicennia zone to the relative high soil temperature that likely caused a high root decomposition rate. The average FRP (3.403–4.079 ton/ha/year) accounted for 74.4, 81.5, and 92.4 % of the total root production in the Avicennia, Rhizophora, and Xylocarpus zone, respectively. The root production and causative factors (i.e., soil temperature and inundation regime) are discussed in relation to the carbon cycle of a mangrove forest.

     

The Impacts of Above- and Belowground Plant Input on Soil Microbiota: Invasive <Emphasis Type="Italic">Spartina alterniflora</Emphasis> Versus Native <Emphasis Type="Italic">Phragmites australis</Emphasis>

Invasive plants affect soil food webs through various resource inputs including shoot litter, root litter and living root input. The net impact of invasive plants on soil biota has been recognized; however, the relative contributions of different resource input pathways have not been quantified. Through a 2 × 2 × 2 factorial field experiment, a pair of invasive and native plant species (Spartina alterniflora vs. Phragmites australis) was compared to determine the relative impacts of their living roots or shoots and root litter on soil microbial and nematode communities. Living root identity affected bacteria-to-fungi PLFA ratios, abundance of total nematodes, plant-feeding nematodes and omnivorous nematodes. Specifically, the plant-feeding nematodes were 627% less abundant when living roots of invasive S. alterniflora were present than those of native P. australis. Likewise, shoot and root biomass (within soil at 0–10 cm depth) of S. alterniflora was, respectively, 300 and 100% greater than those of P. australis. These findings support the enemy release hypothesis of plant invasion. Root litter identity affected other components of soil microbiota (that is, bacterial-feeding nematodes), which were 34% more abundant in the presence of root litter of P. australis than S. alterniflora. Overall, more variation associated with nematode community structure and function was explained by differences in living roots than root or shoot litter for this pair of plant species sharing a common habitat but contrasting invasion degrees. We conclude that belowground resource input is an important mechanism used by invasive plants to affect ecosystem structure and function.     

The ectomycorrhizal community of conifer stands on peat soils 12 years after fertilization with wood ash

We studied long-term effects of fertilization with wood ash on biomass, vitality and mycorrhizal colonization of fine roots in three conifer forest stands growing in Vacciniosa turf. mel. (V), Myrtillosa turf. mel. (M) and Myrtillosa turf. mel./Caricoso-phragmitosa (MC) forest types on peat soils. Fertilization trials amounting 5 kg/m² of wood ash were established 12 years prior to this study. A total of 63 soil samples with roots were collected and analysed. Ectomycorrhizal (ECM) fungi in roots were identified by morphotyping and sequencing of the fungal internal transcribed spacer (ITS) region. In all forest types, fine root biomass was higher in fertilized plots than in control plots. In M forest type, proportion of living fine roots was greater in fertilized plots than in control plots, while in V and MC, the result was opposite. Fifty ECM species were identified, of which eight were common to both fertilized and control plots. Species richness and Shannon diversity index were generally higher in fertilized plots than in control plots. The most common species in fertilized plots were Amphinema byssoides (17.8 %) and Tuber cf. anniae (12.2 %), while in control plots, it was Tylospora asterophora (18.5 %) and Lactarius tabidus (20.3 %). Our results showed that forest fertilization with wood ash has long-lasting effect on diversity and composition of ECM fungal communities.     

Leaf Litter Consumption by Macroarthropods and Burial of their Faeces Enhance Decomposition in a Mediterranean Ecosystem

In many terrestrial ecosystems, large amounts of leaf litter are consumed by macroarthropods. Most of it is deposited as faeces that are easily transferred into deeper soil layers. However, the decomposition of this large pool of organic matter remains poorly studied. We addressed the question of how leaf litter transformation into macroarthropod faeces, and their burial in the soil, affect organic matter decomposition in a Mediterranean dry shrubland. We compared mass loss of intact leaf litter of two dominant shrub species (Quercus coccifera, Cistus albidus) with that of leaf litter-specific faeces from the abundant millipede Ommatoiulus sabulosus. Leaf litter and faeces were exposed in the field for 1 year, either on the soil surface or buried at 5 cm soil depth. Chemical and physical quality of faeces differed strongly from that of leaf litter, but distinctively between the two shrub species. On the soil surface, faeces decomposed faster than intact leaf litter in Quercus, but at similar rates in Cistus. When buried in the soil, faeces and leaf litter decomposed at similar rates in either species, but significantly faster compared to the soil surface, most likely because of higher moisture within the soil enhancing microbial activity. The combined effects of leaf litter transformation into faeces and their subsequent burial in the topsoil led to a 1.5-fold increase in the annual mass loss. These direct and indirect macroarthropod effects on ecosystem-scale decomposition are likely more widespread than currently acknowledged, and may play a particularly important role in drought-influenced ecosystems.     

Nitrogen addition drives convergence of leaf litter decomposition rates between <Emphasis Type="Italic">Flaveria bidentis</Emphasis> and native plant

Evidence is growing that invasive species can change decomposition rates and associated nutrient cycling within an ecosystem by changing the quality of the litter entering a system. However, the relative contribution of their distinct litter types to carbon turnover is less understood, especially in the context of enhanced N deposition. The objective of this study was to investigate the whole-plant responses of an invasive plant Flaveria bidentis in litter decay to simulated N eutrophication. A 1-year study was conducted to assess if N enhancement influenced decomposition and nutrient dynamics of litters from foliage, fine roots and twigs of F. bidentis compared to co-occurring native species Setaria viridis. N fertilization significantly decreased the decomposition rate of the foliage of the invasive F. bidentis by more than 25% relative to the water control, but had relatively minor effects on decomposition of its twigs and fine root litter or leaf litter from the native species. Collectively, decomposition rates of foliar litters of the invasive and native species become convergent over time in the presence of N addition. Moreover, net N loss was predominately influenced by litter species, followed by the litter type, while N addition had little effect on net N loss. Our study showed that the variation in litter decomposition was much greater between litter types of the invasive F. bidentis than between different plant species under the N addition and that the litter of invasive species with higher inherent decomposability did not always decompose more rapidly than the litter of native species in response to predicted N deposition enhancement.     

Leaf C:N:P stoichiometrical and morphological traits of <Emphasis Type="Italic">Haloxylon ammodendron</Emphasis> over plantation age sequences in an oasis-desert ecotone in North China

Haloxylon ammodendron, a typical C₄ desert shrub, is widely used in vegetation reestablishment programs to stabilize shifting sand dunes and to control shifting sands encroaching into oases in the oasis-desert ecotones in northwestern China. In this study, we investigated the morphological traits and analyzed leaf carbon (C), nitrogen (N), and phosphorus (P) stoichiometrical characteristics in an age sequence of 2-, 5-, 9-, 13-, 16-, 31-, and 39-year old H. ammodendron plantations, to assess their growth development and degeneration processes. Results showed that the height, stem diameter, and canopy increased rapidly in the early establishment stage (2–9 years), and thereafter, showed a slower increase from 13 to 39 years. The leaf C and N concentrations and N:P ratio showed a rapid increase from ages 2–5, but C:N exhibited a significant decline. After 5 years, plantation leaf C, N, C:N, and N:P were stable, and leaf P and C:P showed no significant difference among all plantation ages. Leaf C and N concentrations were mainly affected by soil properties. Limiting nutrient factors in soil changed from nitrogen before 2 years to phosphorus after 5 years. The findings suggested that leaf stoichiometrical characteristics combined with morphological traits can be used to reflect the degeneration trend of H. ammodendron. Further research is needed to reveal the processes and mechanisms of H. ammodendron growth development and degeneration, and its adaption to the soil environment.     

Shrub Expansion of <Emphasis Type="Italic">Alnus viridis</Emphasis> Drives Former Montane Grassland into Nitrogen Saturation

The N₂-fixing shrub Alnus viridis is currently encroaching on montane grasslands in the Alps as a result of reduced land management and complete abandonment. Alnus introduces large amounts of nitrogen (N) into these formerly N-poor grasslands and restricts the succession to montane forests. We studied pools and fluxes of N and the associated C pools in pastures (controls) and adjacent Alnus shrublands at two elevations (1650 versus 1950 m a.s.l.) in three valleys in the Swiss central Alps. The total N and C pools stored in 50-year-old Alnus shrubland did not exceed those in adjacent pastures with a total of approximately 610 g N m^?2 in phytomass plus soil (down to 30 cm) at both elevations. In Alnus stands, reduced soil N pools balanced the gain in phytomass N pools, a likely result of a faster turnover of soil N. The soil solution under Alnus was continuously enriched with nitrate, with a total N leaching of 0.79 g N m^?2 season^?1 (June–October) under 50-year-old stands at both elevations and the highest flux of 1.76 g N m^?2 season^?1 in 25-year-old shrubland at low elevation, clearly indicating an excess of available N in Alnus shrubland. In contrast, N leaching across all pastures was close to zero (0.08 g N m^?2) throughout the season. At the catchment scale, streamlet water showed increased nitrate concentrations with typical flushing peaks in spring and autumn, provided more than one fifth of the catchment area was covered by Alnus shrubs. We conclude that the expansion of Alnus rapidly converts centuries-old, N-poor grassland into N saturated shrubland, irrespective of elevation, and it reduces the C storage potential of the landscape because the Alnus dominance constrains re-establishment of a natural montane forest.     

Perennial Grass Bioenergy Cropping on Wet Marginal Land: Impacts on Soil Properties,Soil Organic Carbon,and Biomass During Initial Establishment

The control of soil moisture, vegetation type, and prior land use on soil health parameters of perennial grass cropping systems on marginal lands is not well known. A fallow wetness-prone marginal site in New York (USA) was converted to perennial grass bioenergy feedstock production. Quadruplicate treatments were fallow control, reed canarygrass (Phalaris arundinaceae L. Bellevue) with nitrogen (N) fertilizer (75 kg N ha^?1), switchgrass (Panicum virgatum L. Shawnee), and switchgrass with N fertilizer (75 kg N ha^?1). Based on periodic soil water measurements, permanent sampling locations were assigned to various wetness groups. Surface (0–15 cm) soil organic carbon (SOC), active carbon, wet aggregate stability, pH, total nitrogen (TN), root biomass, and harvested aboveground biomass were measured annually (2011–2014). Multi-year decreases in SOC, wet aggregate stability, and pH followed plowing in 2011. For all years, wettest soils had the greatest SOC and active carbon, while driest soils had the greatest wet aggregate stability and lowest pH. In 2014, wettest soils had significantly (p?<?0.0001) greater SOC and TN than drier soils, and fallow soils had 14 to 20% greater SOC than soils of reed canarygrass + N, switchgrass, and switchgrass + N. Crop type and N fertilization did not result in significant differences in SOC, active carbon, or wet aggregate stability. Cumulative 3-year aboveground biomass yields of driest switchgrass + N soils (18.8 Mg ha^?1) were 121% greater than the three wettest switchgrass (no N) treatments. Overall, soil moisture status must be accounted for when assessing soil dynamics during feedstock establishment.     

Effect of temperate forest tree species on soil dehydrogenase and urease activities in relation to other properties of soil derived from loess and glaciofluvial sand

We investigated the effects of several tree species on dehydrogenase and urease activities in soils derived from two different parent materials (glaciofluvial sand and loess) in forested areas in southern Poland. We hypothesized that coniferous forests (pine, spruce) alter the soil cation exchange capacity (CEC) and decrease soil pH and, therefore, might decrease soil enzyme activities compared with broadleaf species growing on similar soils. Eight paired plots (12 × 12 m) were established on glaciofluvial sand in pine (Pinus sylvestris) + oak (Quercus robur) and spruce (Picea abies) + pine stands, as well as on loess-derived soils: beech (Fagus sylvatica) + pine and hornbeam (Carpinus betulus) + pine stands. Each plot was a 4 × 4 m grid with 16 sampling points. In soil samples pH, soil texture, and organic carbon, nitrogen, base cation contents, dehydrogenase and urease activities were determined. On both parent materials, the soil pH was lower under coniferous species than under broadleaf species. The acidifying effect of tree species on sandy soil was in the order of spruce = pine > oak, while that on loess was pine > beech > hornbeam. Hornbeam and oak increased the soil pH and stimulated enzyme activity in the soil. The content of fine fraction enhanced potential enzyme activities in soils, thus the loess soils had greater dehydrogenase and urease activity. The results suggest that pine stores more soil organic C in association with silt increasing the pool of stabilized soil organic C.